Standard image sensors have a limited dynamic range of about 60 to 70 dB or less. For example, an 8-bit sensor has a dynamic range of only 48 dB. However, the luminance dynamic range of the real world is much larger and natural scenes often span a range of 90 dB or more. When an image sensor captures a scene with a luminance dynamic range in excess of the sensor dynamic range, information is necessarily lost. Depending on the exposure settings, the brighter regions may be saturated and/or the dimmer regions may be underexposed, producing a captured image of a quality unable to reproduce the actual scene.
In order to simultaneously capture highlights and shadows of a scene, image sensors have used High Dynamic Range (HDR) technologies to increase the captured dynamic range. One of the most common techniques to increase the dynamic range is to merge multiple exposures, captured with a standard, low dynamic range image sensor, into a single HDR image that has a much larger dynamic range than a single exposure image. For instance, images of the same scene may be recorded at two different exposure times, where the longer exposure is set to optimally capture the dimmest portions of the scene and the shorter exposure is set to optimally capture the brightest portions of the scene. However, such image combination methods frequently suffer from artifacts caused, for example, by objects in the scene moving between exposures.
HDR image sensors that capture an HDR color image in a single frame have been developed. Such an HDR image sensor includes two interleaved pixel arrays, one pixel array with larger pixels and another pixel array with smaller pixels. The larger pixels are more light sensitive, thus optimal for capture of dim scenes or dim portions of a scene. The smaller pixels are less light sensitive, thus optimal for capture of bright scenes or the brighter portions of a scene.